1. Field of the Invention
This invention relates to an electroless Nixe2x80x94B plating liquid, an electronic device and a method for manufacturing the same. More particularly, this invention relates to an electroless Nixe2x80x94B plating liquid useful for forming a protective film for protecting the surface of the interconnects of an electronic device which has such an embedded interconnect structure that an electric conductor, such as silver or copper, is embedded in fine recesses for interconnects formed in the surface of a substrate such as a semiconductor substrate, and to an electronic device having the interconnects-protecting film formed by using the plating liquid, and a method for manufacturing the same.
2. Description of the Related Art
As a process for forming interconnects in an electronic device, the so-called xe2x80x9cdamascene processxe2x80x9d which comprises filling trenches for interconnects and contact holes with a metal (electric conductor), is coming into practical use. According to this process, aluminum or, more recently a metal such as silver or copper, is filled into trenches for interconnects and contact holes previously formed in the interlevel dielectric of a semiconductor substrate. Thereafter, an extra metal is removed by chemical mechanical polishing (CMP) so as to flatten the surface of the substrate.
In the case of interconnects formed by such a process, the embedded interconnects have an exposed surface after the flattening processing. When an additional embedded interconnect structure is formed on such an exposed surface of the interconnects of a semiconductor substrate, the following problems may be encountered. For example, during the formation of a new SiO2 in the next interlevel dielectric forming process, the exposed surface of the pre-formed interconnects is likely to be oxidized. Further, upon etching of the SiO2 film for formation of via holes, the pre-formed interconnects exposed on the bottoms of the via holes can be contaminated with an etchant, a peeled resist, etc.
In order to avoid such problems, it has conventionally been performed to form a protective film of SiN or the like not only on the interconnect region of a semiconductor substrate where the interconnects are exposed, but on the whole surface of the substrate, thereby preventing the contamination of the exposed interconnects with an etchant, etc.
However, the provision of a protective film of SiN or the like on the whole surface of a semiconductor substrate, in an electronic device having an embedded interconnect structure, increases the dielectric constant of the interlevel dielectric, thus inducing delayed interconnection even when a low-resistance material such as silver or copper is employed as an interconnect material, whereby the performance of the electronic device may be impaired.
In views of this, it may be considered to selectively cover the surface of the exposed interconnects with a Nixe2x80x94B alloy film having a good adhesion to an interconnect material such as silver or copper and having a low resistivity (xcfx81). A plated Nixe2x80x94B film, obtained by electroless Nixe2x80x94B plating, is either a crystalline or an amorphous plated film depending on the boron content of the film. In this regard, a crystalline plated film is obtained when the boron content of the film is less than 10 at % (atomic %), and an amorphous plated film is obtained when the boron content of the film is 10 at % or more, generally.
When a plated Nixe2x80x94B film is used for the purpose of protecting the interconnects of an electronic device having an embedded interconnect structure, the plated film is required to be thermally stable. From this point of view, it is necessary to use a crystalline plated film having a boron content of less than 10 at %. This is because a crystalline plated Nixe2x80x94B film maintains its crystallinity after a heat treatment, whereas an amorphous Nixe2x80x94B plated film forms a Nixe2x80x94B compound upon the heat treatment and thus becomes an unstable film.
However, when an intended crystalline Nixe2x80x94B film, for the purpose of protecting the interconnects of an electronic device having an embedded interconnect structure, is formed by electroless plating by using a plating liquid that is formulated to provide a plated film having a lowered boron content, the plating rate is likely to become too high to make a proper control of the process.
In this regard, in electroless plating, the reaction time is equal to the solid-liquid contact time between the plating liquid and an object to be plated. Further, a plated Nixe2x80x94B film to be used for protecting the interconnects of an electronic device must be as thin as several tens to several hundreds nm. Accordingly, an enhanced plating rate makes the process control more difficult.
The present invention has been made in view of the above situation in the related art. It is therefore an object of the present invention to provide an electroless Nixe2x80x94B plating liquid which can lower the boron content of the resulting plated film without increasing the plating rate and form a Nixe2x80x94B alloy film having an FCC (face centered cubic) crystalline structure, and also to provide an electronic device in which the interconnects are protected with the plated film formed by electroless plating carried out by using the plating liquid, and a method for manufacturing the same.
In order to achieve the above object, the present invention provides an electroless Nixe2x80x94B plating liquid for forming a Nixe2x80x94B alloy film on at least part of interconnects of an electronic device having an embedded interconnect structure, the electroless Nixe2x80x94B plating liquid comprising nickel ions, a complexing agent for the nickel ions, a reducing agent for the nickel ions, and ammoniums (NH4+).
The inclusion of ammonums (NH4+) in the plating liquid can lower the boron content of the plated film to provide a Nixe2x80x94B alloy film having an FCC crystalline structure, and can also lower the plating rate by ammoniums (NH4+) so as to thereby facilitate the process control. It is considered, in this regard, that an ammonia ion, due to its generally high chelating force, may form a complex with a nickel ion to thereby lower the plating rate.
The reducing agent may be, for example, an alkylamine borane or a hydrogen boride compound. Specific examples of the alkylamine borane include dimethylamine borane, diethylamine borane and trimethylamine borane. NaBH4 may be mentioned as a specific example of the hydrogen boride compound.
The ammonums may be prepared from e.g. ammonia water.
The pH of the electroless Nixe2x80x94B plating liquid may be adjusted within the range from 8 to 12. By thus increasing the pH of the plating liquid to 8-12, it becomes possible to lower the boron content of the plated film and form a Nixe2x80x94B alloy film having an FCC crystalline structure. The pH of the plating liquid is preferably 9-12, more preferably 10-12.
The temperature of the electroless Nixe2x80x94B plating liquid may be adjusted within the range from 50xc2x0 C. to 90xc2x0 C. To raise the liquid temperature to 50xc2x0 C. or higher promotes the plating reaction, whereas to control the liquid temperature to 90xc2x0 C. or lower prevents an increase in the boron content of the plated film. The temperature of the plating liquid is preferably adjusted to 55-75xc2x0 C.
The present invention also provides an electronic device having an embedded interconnect structure of silver, silver alloy, copper or copper alloy, wherein a surface of an interconnect is selectively covered with a protective layer of a Nixe2x80x94B alloy film.
By thus selectively covering the surface of the interconnects and protecting the interconnects with the protective film of a Nixe2x80x94B alloy film that has a high adhesion to silver or copper and has a low resistivity (xcfx81), an increase in the dielectric constant of the interlevel dielectric of an electronic device having an embedded interconnect structure can be suppressed. Further, the use as an interconnect material of a low-resistance material, such as a silver or copper, can attain speedup and densification of the electronic device.
The present invention further provides a method for manufacturing an electronic device, comprising; electroless plating an electronic device having an embedded interconnect structure with an electroless Nixe2x80x94B plating liquid to form a protective layer of a Nixe2x80x94B alloy film selectively on a surface of an interconnect of the electronic device; wherein the electroless Nixe2x80x94B plating liquid comprises nickel ions, a complex agent for nickel ions, a reducing agent for nickel ions, and ammonums (NH4+).
Plating with an electroless Nixe2x80x94B plating liquid containing an alkylamine borane or a hydrogen boride compound as a reducing agent, e.g. an electroless Nixe2x80x94B plating liquid containing as a reducing agent DMAB (dimethylamine borane) that causes an anodic oxidation reaction with silver, is known to be effected selectively onto silver or copper. Thus, by immersing the substrate of an electronic device having an exposed surface of interconnects in the plating liquid, plating is effected selectively onto the exposed surface of the interconnects.
The above and other objects, features, and advantages of the present invention will be apparent from the following description when taken in conjunction with the accompanying drawings which illustrates preferred embodiments of the present invention by way of example.